What I'm getting at there are alot of variables especially when it comes to t-5's. Rules of thumb are a bad idea. If you are going to keep SPS with T-5's I would do as much research as possible on bulbs, spectrum, par, ballast, reflectors and stuff like overdriving as possible.
The evidence I have seen sudgest that with the right equipment and overdriving t-5's can penetrate as deep and deeper than halides. For instance I have a 24' deep 120 with a 2x250 hqi setup. My average par is just under 300 at my sandbed. Another friend of mine has a 150 with the same demensions just deeper that is running a t-5 setup. Our wattages are almost equivelent (his is actually a little lower) and he averages around 400 in a deeper tank.